Exporting Cycles Sky to skybox images
You can UVmap a cube and add a Glossy Material to it.  Set the Roughness parameter to 0.0
Add a Sub-Surface Modifier to the cube and set it to about 4 iterations, this helps for baking the Glossy Shader properly.

Add a texture node to the material so the sky can be baked to the UV Coordinates.



Now invert the faces for the cube so they face inwards. This should work as a SkyBox. 



If you want the sky to be available in a GLSL shader as a reflection map then you may want to pay closer attention to how the UV's are laid out since a game engine may have specific layout requirements for skybox/enviroment/cubeMaps.

Here's some reference on how to code a cubemap using a runtime technique that was published by nVidia, it uses nVidia CG which is a shader language that is compatible with both OpenGL and DirectX.  

https://web-beta.archive.org/web/20160408223256/http://http.developer.nvidia.com:80/CgTutorial/cg_tutorial_chapter07.html

And the source code to go along with the CgTutorial

https://developer.nvidia.com/cg-toolkit-download